1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a toner image transfer type electrophotographic image forming apparatus.
2. Description of Related Art
In the art of toner image transfer type electrophotographic copying apparatus, a toner image formed on a photosensitive member is transferred to a copy sheet by applying charge which has polarity opposite to the polarity of the toner to the copy sheet on the back side by a transfer charger, or in a full color copying apparatus, the toner image is once transferred to an intermediate transfer belt or drum, and then transferred to the copy sheet.
As for the toner image transfer to the copy sheet, though the transfer efficiency is hoped to be 100%, practically, it is deteriorated by environmental conditions (humidity, etc.), the material and the thickness of the copy sheet, and the like. Fine transfer efficiency can be maintained by controlling the output of the transfer charger, but minute adjustment of the transfer charger to the environmental conditions and the characteristics of the copy sheet is very difficult. Especially in the copying apparatus using the intermediate transfer belt, the problem appears clearly because the performance of the intermediate transfer belt comparatively depends on the environments.
In order to keep fine transfer efficiency, detecting the thickness of the copy sheet (Japanese Patent Laid Open Publication No. 58-17468), detecting a time constant of the copy sheet (Japanese Patent Laid Open Publication No. 58-60756), and detecting the humidity (Japanese Patent Laid Open Publication No. 60-44662) have been proposed.
However, the transfer efficiency can not be well controlled only by changing the output of the transfer charger according to the environmental conditions and the characteristics of the copy sheet. Also according to the result of the experiment, when an electric current flowing from the transfer charger to a toner image carrying member, namely the photosensitive member or the intermediate transfer belt, is an appropriate value, fine transfer efficiency is ensured regardless of the output (applied voltage) of the transfer charger.